The present invention relates to an apparatus for controlling fusing temperature in an electrographic developing system, and more particularly, to an apparatus and method for controlling fusing temperature at the touch of a key in accordance with the thickness of the paper.
Generally electrophotographic developing systems consist of a laser beam printer, an electrophotographic copying machine or the like. The operation of the laser beam printer is disclosed in Korean Patent application No. 92-11243. In such a printer, the toner attached to recording paper by the developing process is fused to the paper by a fusing device. The fusing device utilizes a heated heat roller and a pressing roller. The heat provided by the heat roller is generated from a fusing lamp installed in the interior of the heat roller. The fusing temperature, or temperature of the heat produced by the heat roller, is determined by the heat generated from the fusing lamp. Consequently, when recording paper is jammed in the fusing device, high fusing temperature may cause the jammed paper to catch fire. Alternatively, if the fusing temperature is too low, the toner may not be properly fused to the recording paper.
A conventional apparatus for controlling fusing temperature incorporates a temperature sensing device, a set temperature control device, a CPU and a fusing lamp driving device. The temperature sensing device consists of a thermistor, a resistor and a variable resistor, for generating a temperature sensing voltage in accordance with the fusing temperature and changes in fusing temperature. The set temperature control device consists of a comparator for comparing the temperature sensing voltage of the temperature sensing device with a temperature set voltage produced by a pair of resistors, and a feedback resistor connected to a terminal and the output terminal of a comparator. The CPU receives the set temperature control signal outputted from the set temperature control device to generate a fusing temperature control signal. The fusing lamp driving device consists of a group of resistors, a phototriac, a transistor and capacitors for driving a fusing lamp in response to the fusing temperature control signal outputted from the CPU.
The set temperature control device controls the fusing temperature according to a set temperature corresponding to the temperature set voltage provided by the resistors. The set temperature is fixed regardless of the kind of the paper. Therefore, when the temperature sensing voltage of the temperature sensing device, which senses the heating temperature of the fusing lamp, is over the set temperature produced by the resistors, the output of the comparator shifts. At this time, the CPU turns off the transistor, thereby disabling the phototriac.
When the phototriac is turned off, the power supply of the fusing lamp is cut off. Consequently, the fusing lamp is no longer heated and the fusing temperature drops. After the fusing lamp remains off for a period of time, the temperature sensing voltage drops below the set temperature corresponding to the temperature set voltage, rendering the comparator to again shift its output or set temperature control signal. At this time, the CPU turns on the transistor and the phototriac. When the phototriac is turned on, the fusing lamp starts to generate heat to raise the fusing temperature. If it is not necessary to control the fusing temperature, the CPU disables the power supply to the fusing lamp.
In addition to the conventional apparatus explained above, other apparatus for controlling fusing temperature exist.
Itoh, U.S. Pat. No. 4,373,801 entitled Fixing Temperature Selecting Control in a Copying Machine, discloses a device which controls fusing temperature according to the thickness of a sheet of paper. Two manual switches disposed upon the device produce signals which change the fusing temperature for a thick sheet of paper or a thin sheet of paper. The device also contains two inlets. One for receiving thin paper and one manual inlet for receiving thick paper.
Hanamoto et al., U.S. Pat. No. 4,439,143 entitled Heat Rolling Fixing Device, discloses a fixing device which varies temperature according to the thickness of copy paper. This device merely switches between two fusing temperatures, one temperature for thick copy paper and one temperature for normal copy paper. These respective temperatures are obtained by activation of a manual switch disposed upon the device.
Nakajima et al., U.S. Pat. No. 5,307,134 entitled Electrophotographic Apparatus, discloses a fixing device wherein the fixing temperature can be changed in accordance with the thickness of a sheet of paper. This device utilizes two paper paths to distinguish between a normal sheet of paper and a thick sheet of paper. Accordingly, the user must enter a sheet of paper into one of the two paper paths and the device changes fixing temperature according to the guide path which the paper travels through.
Rohrer et al., U.S. Pat. No. 4,835,573 entitled Machine Control System Utilizing Paper Parameter Measurements, discloses a method and apparatus for calculating sheet thickness. This method and apparatus uses a "sheet quality factor" determined by dividing the amount in height a stack of paper changes in response to withdrawal therefrom of a predetermined number of sheets. After the "sheet quality factor" is determined and the thickness of the sheets are obtained, the device suggests adjusting the temperature of the image fuser correspondingly.
In the conventional apparatus for controlling fusing temperature, the fusing temperature is maintained at a constant level. Consequently, when fusing toner to a thin sheet of paper at an excessively high temperature, the paper may be crumpled when ejected, catch on a heat roller, or jam during the printing operation. Alternatively, when fusing toner to a thick sheet of paper, the fusing operation is not normally completed since the temperature in the heat roller becomes drastically lower due to loss of heat caused by the thickness of the sheet of paper.
Moreover, the conventional apparatus which attempt to control fusing temperature do not calculate the actual thickness of each sheet of paper in order to properly control the fusing temperature of the fusing device. These devices utilize either a manual method of determining the thickness of the sheet of paper or a method which determines average sheet thickness by calculating "a sheet quality factor". Additionally, these devices only contemplate temperature variations for two thickness. Consequently, wide variations of paper thickness must be termed either thick or normal and the fusing temperature set accordingly. This may still result in the aforementioned problems of paper crumpling or inadequately fusing the toner to the paper in the case of overly thin sheets of paper and overly thick sheets of paper.